Wireless communication systems contain subscribers, which may be mobile or portable radio units, and fixed infrastructure through which the subscribers communicate in a trunked mode. The infrastructure contains, for example, base stations and controllers. The subscribers are geographically distributed and generally communicate using different base stations.
Typical communications systems transmit voice, video or other data (hereinafter referred to merely as “data”) messages amongst the subscribers and base stations using working channels, which are predetermined frequencies and timeslots. The working channels are random access channels and are different dependent on the message direction: an inbound (or uplink) channel is used for communications from the subscriber to the base station and an outbound (or downlink) channel is used for communications from the base station to the subscriber. When a subscriber wishes to transmit data to other subscribers, the subscriber first determines the status of the uplink channel—i.e., whether it is busy or idle. In European Telecommunications Standards Institute Digital Mobile Radio (ETSI-DMR) systems, the downlink channel periodically transmits a CACH (Common Announcement Channel) burst that indicates the status of the channel.
In many systems, the subscriber is required to monitor the uplink channel for an extended period of time prior to attempting to transmit data. Once the subscriber determines that the uplink channel is idle, the subscriber may attempt to transmit the data by first sending a request to transmit to the base station. If a large number of subscribers use the same working channel, multiple subscribers may attempt to transmit these requests at the same time, causing collisions between the requests. Base stations receiving multiple colliding messages at the same time typically do not respond to the messages as they mutually interfere with one another, causing each message to be retransmitted. Adding to this, communication systems also typically require a confirmation message be sent to the subscriber on the outbound channel to confirm receipt of the message from the subscriber. This increases the bandwidth usage on the outbound channel as well as further increasing the amount of time it takes to transmit the data from the subscriber.
These problems have become increasingly problematic due to the recent desirability of determining the location of the subscriber using the Global Positioning System (GPS) or other systems. As subscriber location information presents a heavy traffic load on a channel due to its frequent transmissions, to minimize the impact that location data might have on other data traffic, such as voice traffic, the location data can be transmitted on a separate random access channel. The throughput of this separate channel, however, is also limited by the above factors, i.e., channel access procedures (which take about 540 ms) and collision probability. Simulation in one example (in which ETSI DMR protocol using half rate FEC, transmissions are 6 bursts and radios have a 150 ms channel access collision window is used) shows that when sending location messages on the separate channel, for a target probability of success to be 93% or better (which is generally considered an acceptable level), no more than 20 updates per minute per channel should be attempted. While this is reasonable when small groups of subscribers are being tracked, it becomes exceedingly problematic if it is desired to track the locations of a large number of subscribers (e.g., greater than several hundred). This situation may occur, for example when governmental agencies (federal, state, or local) wish to track assigned communication devices in emergency service or other government vehicles. Moreover, other industries such as transportation (e.g., trucking), utilities, manufacturing, hospitality, retail, airport, construction, private security, or storage may wish to employ such tracking. In this case, however, the amount of infrastructure equipment employed to provide the tracking while providing acceptable level of success for location requests becomes large and correspondingly costly.
As it is likely that the desirability for location tracking will only increase, and therefore the number of devices being tracked correspondingly increase, it is therefore desirable to provide a method and system for location tracking using the data revert channel(s) in which number of revert channels and the amount of infrastructure employed, and thus cost, is reduced and in which the amount of time for channel access is minimized.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments shown so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Other elements, such as those known to one of skill in the art, may thus be present.